Lines of clone restriction, called compartmental boundaries, arise successively in Drosophila melangaster and subdivide the blastoderm stage embryo and later the different imaginal discs. Although clonal isolation cannot prove the isolated populations of cells have different commitments, sequential compartmentalization suggests commitments arise sequentially, and that each terminal compartment is specified by the particular combination of alternative commitments made during its formation in an epigenetic code. We have analyzed the onset of the earliest clone restrictions using X-ray induced somatic recombination. One restriction line appears to isolate the anterior from posterior egg during the late syncytial blastoderm, and others arise sequentially to segment the embryo longitudinally. We shall reanalyze this using a mutant which produces spontaneous clones throughout cleavage. We shall test whether clonal isolation reflects developmental commitments by transplanting blastoderm cells, or nuclei and cytoplasm from the late syncytial stage, to homotopic and heterotopic positions on genetically marked, synchronous hosts. Homotopic transplantations will produce a direct fate map of the blastoderm, and show the developmental potential of single blastoderm cells. Transplantation within a compartment should result in integration into host site structures, while transplantation across compartmental boundaries should yield autonomy. Through analysis of physical sorting out of transplants in the embryo, and the patterns of donor site, host site, or allotypic structures in adult mosaics, we shall attempt to assess the compartmental hypothesis, the combinatorial code model, and the possibilities that qualitatively different determinative factors or monotonic gradients of morphogens trigger early commitments.